A starter of this type comprises a support, an electric motor which is arranged on the support and which serves for driving a pinion in rotation, and a solenoid drive which is arranged on the support and which serves for the axial adjustment of the pinion between an engagement position, which is provided for the drive of a gearwheel of the internal combustion engine, and a non-engagement position, which is offset axially with respect to the engagement position.
The solenoid drive used here comprises a ferromagnetic housing and a cylindrical coil arrangement which has at least one electric coil, wherein the coil arrangement is arranged in the housing and coaxially surrounds a cylindrical coil interior space. Furthermore, a ferromagnetic plunger stop is provided which is arranged at a first axial end of the coil arrangement in the housing and which has a central region projecting axially into the coil interior space. Finally, a ferromagnetic plunger is provided which, at a second axial end of the coil arrangement, which axial end is opposite the central region of the plunger stop, projects axially into the coil interior space, and which is arranged so as to be adjustable axially bi-directionally relative to the housing between an active position which is proximal with respect to the central region and a passive position which is distal with respect to the central region. The drive coupling between plunger and pinion takes place in such a manner that, in the passive position of the plunger, the pinion is in the non-engagement position while said pinion is transferred into the engagement position thereof by adjustment of the plunger into the active position.
For the starting of the internal combustion engine, the solenoid drive is activated so as to transfer the pinion of the starter from the non-engagement position into the engagement position. For this purpose, the plunger is adjusted from the passive position into the active position. In the engagement position, the pinion meshes with a gearwheel of the internal combustion engine, which may be formed for example on a flywheel of a drive train of the internal combustion engine. The electric motor then drives the pinion, which in turn drives said gearwheel, as a result of which a crankshaft of the internal combustion engine is set into rotation in order to start the internal combustion engine. As soon as the internal combustion engine has started and the crankshaft thereof is driven by reciprocating movements of pistons of the internal combustion engine, the solenoid drive is activated such that the pinion is returned again from the engagement position into the non-engagement position. For this purpose, the plunger is adjusted back from the active position into the passive position. In the non-engagement position, the pinion disengages from said gearwheel, that is to say no longer meshes with the latter.
In order to be able to adjust the pinion from the non-engagement position into the engagement position and in order to be able to secure the pinion in the engagement position, the coil arrangement has to transmit comparatively large electromagnetic forces to the plunger in order to draw the latter into the coil interior space and hold said plunger therein, for the active position. Since, for the purposes of a failsafe design, the plunger is preferably drawn into the coil interior space counter to the action of a restoring spring, comparatively high magnetic forces are required in particular to hold the plunger static in the active position, and therefore the coil arrangement is supplied with a correspondingly high level of electrical power.
The pinion normally has a circumferential toothing with axially extending teeth. Complementary with respect thereto, the gearwheel of the internal combustion engine likewise has a circumferential toothing with axially running teeth. Upon a transfer of the pinion from the non-engagement position into the engagement position, the teeth of the pinion engage in toothed spaces of the gearwheel. However, in many situations, axially leading tooth flanks of the teeth of the pinion do not pass directly into the toothed spaces of the toothing of the gearwheel but strike against axial tooth flanks of the teeth of the gearwheel. In order that the teeth of the pinion nevertheless find the toothed spaces of the gearwheel and can engage therein, the electric motor of the starter may be activated so as to effect a rotation of the pinion as early as during the adjustment of the pinion from the non-engagement position into the engagement position. Said rotation for the threading-in of the pinion into the gearwheel is expediently performed with a considerably reduced torque and/or with a considerably reduced rotational speed in relation to the subsequent starting operation, when the pinion is fully engaged with the gearwheel.
For said two-stage starting operation, which may also be referred to as “soft-start”, in the case of a starter of this type an electric series connection of the electric motor and of the solenoid drive is expediently proposed, and therefore, for the reduced driving of the electric motor, the voltage provided for energising the coil arrangement can be used in conjunction with the associated current. The solenoid drive then serves at the same time as a switch for connecting the electric motor to the actual motor current supply. In this respect, the solenoid drive at the same time forms an electromagnetic switch.
Owing to the above-described, comparatively high magnetic force with which the plunger is drawn into the coil interior space, the pinion may, by way of the axially leading tooth flanks thereof, collide with the opposite axial tooth flanks of the gearwheel with corresponding intensity, increasing the wear of the toothings of pinion and gearwheel. Furthermore, the toothings may bear against one another via the axial tooth flanks with a comparatively high force, as a result of which a correspondingly high level of friction has to be overcome in order to rotate the pinion relative to the gearwheel such that the toothing of the pinion can mesh with the toothing of the gearwheel. As a result, there is the risk of increased wear here too.
A starter of this type is known, for example, from U.S. Pat. No. 8,421,565 B2. To solve the above mentioned problem, in the case of the starter, said document proposes a complex construction of the coil arrangement within the solenoid drive, wherein a retraction coil for pulling the plunger into the coil interior space and a holding coil for holding the plunger that is being pulled into the coil interior space are arranged axially separately from one another. It is also proposed that the plunger be equipped, on the outer circumference thereof, with an encircling annular groove which, in the passive position, is situated radially opposite an edge region circumferentially surrounding a passage opening, through which the plunger passes axially, of an end side wall of a solenoid housing. In this way, in the passive position, there is a radial gap between plunger and edge region. As the plunger is retracted into the coil interior space, the circumferential groove moves into the coil interior space and thereby departs from the above mentioned edge region of the end side wall, such that said edge region is subsequently situated radially opposite a plunger longitudinal section axially adjoining the circumferential groove. As the plunger is retracted, therefore, a radial distance between said edge region and an outer side of the plunger is varied, specifically reduced, as a result of which the density of the magnetic field lines transmitted from said edge region to the plunger when the coil arrangement is switched on, is varied, specifically increased. However, the density of the magnetic field lines correlates with the acting magnetic forces. The circumferential groove formed on the plunger thus yields a reduction in the acting magnetic forces at the start of the retraction movement of the plunger when the pinion is to be transferred from the non-engagement position into the engagement position. Said known measures are, however, relatively cumbersome to realise. Furthermore, the attractive force that pulls the plunger into the coil interior space is reduced only to a comparatively small extent by the annular groove, since said annular groove ultimately merely effects a deflection of the field lines. Also, the annular groove is maintained and, even when the plunger has been retracted into the coil interior space, causes a deflection of the field lines in the plunger, thus reducing the attainable magnetic forces.
DE 10 2009 052 938 A1 discloses another solution to this problem. In this document, the solenoid drive, which is referred to as an electromagnetic switch, is equipped with a ferromagnetic bypass device, which, when the coil arrangement is energized, diverts some of the magnetic field lines directly from the plunger into the plunger stop, at least in the passive position of the plunger, such that said field lines do not extend through an air gap formed axially between the plunger and the plunger stop. Since, however, the field lines extending through said air gap are crucial for the magnetic force which drives the plunger into the coil interior space, the force acting on the plunger may be reduced for the beginning of the adjustment movement. With increasing penetration depth of the plunger into the coil interior space, the diversion of the magnetic field lines by the bypass device is reduced, as a result of which the magnetic force driving the plunger increases. It has even been shown that, in the active position, the magnetic holding force which holds the plunger in the active position can be increased with the aid of such a bypass device. The same then holds true for the forces which act on the pinion and drive the pinion from the non-engagement position into the engagement position and optionally hold said pinion therein. In this known configuration a part of the magnetic flux is bypassing the axial gap between plunger and plunger stop by passing directly from the housing via the bypass device to the plunger stop. Therefore, the exact axial position of the bypass device relative to the housing and relative to the plunger stop is essential for the deviating effect. Accordingly, narrow production tolerances have to be used.
In the case of the known solenoid drive, the bypass device is formed by a ferromagnetic annular body which is dimensioned and arranged in the coil interior space in such a manner that said annular body extends as far as the second axial end of the coil arrangement and is supported there preferably on the housing and is in contact therewith.